In a conventional technique, there are known machine tools that relatively move tools to workpieces so as to perform machining such as cutting. In addition, among such machine tools, there is known a machine tool of a numerical control type that designates a path of the tool based on coordinates or the like of a predetermined feed axis and performs machining while moving the tool relative to the workpiece. By moving at least one of the workpiece and the tool according to a command from a control apparatus, the machine tool can automatically perform machining while changing a relative positon of the tool with respect to the workpiece.
When creating a tool path that determines a relative positon of the tool with respect to the workpiece, a shape of the tool is taken into consideration. For example, a path through which the tool moves is set with a tool center used as a reference. The tool center is set on a center axis of the tool, but it is a surface of the tool that actually performs machining. Accordingly, the tool path is created with consideration given to a tool diameter. However, there is an individual difference in shape among tools, and an error occurs with regard to a basic size of the tool. Accordingly, certain machine tools may include tool measurement apparatuses that measure tool sizes such as a tool length and a tool diameter.
Japanese Unexamined Patent Application Publication No. 8-229776 discloses a machine tool that includes a measurement apparatus which measures a tool length by a displacement measuring device of electrostatic capacitance type. In this machine tool, a tool in the midst of rotation at a rotational speed for actual machining is caused to approach a measurement electrode of the displacement measurement device. Position data of a feed axis when a gap between the tool and the measurement electrode reaches a predetermined reference value is detected by a position detection unit. Then, it is disclosed that an arithmetic unit fetches tool data stored in advance in a tool data storage unit, and calculates cutting edge position displacement of the tool based on the position data and the tool data.